The heterogeneity of leukemias has been unmasked by probes designed to identify proteins in individual cells. This experimental approach has led to major advances in the study of differentiation in normal hematopoiesis. We have purified, characterized, and prepared antibodies to three human enzymes that are markers of lymphoid differentiation. We will now pursue biochemical questions about protein processing and structural relationships of these proteins to tumor antigens and to other enzymes. We have three specific aims in mind: (1)\We are continuing to develop monoclonal antibodies to TdT and ADA. The monoclonal antibodies have been used to develop immunoperoxidase methods of protein detection and have been used for the construction of affinity columns for rapid protein purifications. The antibodies are also being used in binding studies to dissect the structures of multiple forms of the two proteins and to assess structural relationships with other proteins. (2)\We are cloning cDNAs coding for human ADA. Using probes to study levels of mRNA coding for ADA, we are constructing an expression vector for ADA peptides and have begun fluorescence studies of human ADA protein. (3)\We are beginning studies to purify human MTA-Pase to homogeneity. (B)